News Release Number: STScI-2000-33

Astronomers Ponder Lack of Planets in Globular Cluster

October 31, 2000: Astronomers using the Hubble telescope made the first broad search for planets
far beyond our local stellar neighborhood. They trained Hubble's "eagle eye" for
eight days on a swarm of 35,000 stars in 47 Tucanae, located in the southern constellation
Tucana. The researchers expected to find 17 "extrasolar" planets. To their surprise,
they found none. These results may be the first evidence that conditions for planet
formation and evolution are different in other regions of our Milky Way Galaxy.

Q & A: Understanding the Discovery

1.
Why didn't astronomers find any planets?

Astronomers are pondering several possibilities for 47 Tucanae's dearth of planets.

Possibility #1: The stars they observed were in a globular cluster, a compact region of up to 1 million middle-aged stars. Globular clusters lack the heavier elements that may be necessary for building planets.

Possibility #2: Astronomers used Hubble to hunt for a specific type of planet called a "hot Jupiter," a bloated, Jupiter-sized planet that snuggles perilously close to its parent star. The results do not rule out the possibility that 47 Tucanae could contain normal solar systems like ours, which Hubble cannot detect. But even if that's the case, the finding implies there is a fundamental difference between the way planets are created in our own neighborhood and how they're made in the cluster.

Possibility #3: The stars are so tightly compacted in the core of the cluster that strong gravitational forces may strip nascent planets from their parent stars. The high volume of stars also could disturb a Jupiter-size planet's migration inward towards its parent star. Astronomers were looking for these "star huggers," called hot Jupiters, in 47 Tucanae. Scientists theorize that many Jupiter-size planets found orbiting close to their host stars actually formed farther away and traveled inward.

Possibility #4: A torrent of ultraviolet light from the cluster's earliest and biggest stars, which were born billions of years ago, may have boiled away fragile embryonic dust disks from which planets would have formed.

2.
How did the Hubble telescope look for planets?

The telescope didn't snap pictures of planets: They're too small and dim to be imaged by any observatory. Astronomers used the Hubble telescope's Wide Field and Planetary Camera 2 to search for small dips in starlight, the telltale signature of planets passing in front of stars. As a planet crosses a star's face, it blocks a tiny amount of starlight. This event is similar to the moon eclipsing our Sun.